In recent years, it has been proposed to form a heat diffusion member for cooling a heat source such as a semiconductor element of a carbon base material such as graphite, carbon fiber, a carbon-metal composite, or diamond, in place of a metallic material such as copper and aluminum.
For example, JP2009-4666A, which corresponds to US2008/0315401A1, describes to employ a heat diffusion member made of a carbon fiber-metal composite between a semiconductor element and a heat radiation plate. The heat diffusion member is disposed such that an orientation direction of carbon fiber coincides with a thickness direction of the heat diffusion member, so as to enhance heat conduction from the semiconductor element to the heat radiation plate. The thickness direction means a direction in which a thickness of the heat diffusion member is measured.
The carbon fiber has a coefficient of thermal conductivity of approximately 1000 W/mk in the orientation direction. Therefore, the heat diffusion member has a coefficient of thermal conductivity higher than that of copper (e.g., 350 to 400 W/mk) and that of aluminum (e.g., 200 to 240 W/mk) in the orientation direction.
In JP2009-4666A, however, an Ag—Cu—In base brazing material or an Ag—Cu—In—Ti base brazing material is used to bond the heat diffusion member to an adjacent member. Therefore, a bonding temperature (e.g., bonding temperature) is high, such as 500 to 800 degrees Celsius, and hence a thermal stress due to a difference of coefficient of linear expansion between the carbon-based heat diffusion member and the adjacent member is likely to increase. If the thermal stress, that is, a residual stress at the bonding interface is large, a breakage will easily occur at the bonding interface during an operation of the semiconductor element.